Ethylene concentrations more than 10 ppbv (part per billion volume) may have an adverse influence on the freshness and stock life time of perishable produce like fruit, vegetables as well as flowers and living plants. The produce may emit ethylene when it ripens, which may influence other produce in the vicinity leading to decay and reduced stock life during storage or shipping. This is the cause of a great amount of waste of produce that has become unmarketable, which is both a great environmental and economic challenge. Therefore, there is a demand for systems which effectively and economically profitably can reduce and/or remove ethylene or other organic compounds polluting the environment in storing and/or transport systems for such produce, such as e.g. reefer containers and refrigerators with controlled internal atmospheres.
Known methods of ethylene removal are ventilation, scrubbing in potassium permanganate scrubbers, and chemical reactions using ozone. Adequate ventilation may be unacceptable, because it may influence other control parameters of the controlled internal atmosphere, such as e.g. temperature, humidity, CO2 and O2 and will increase energy consumption in cooled and temperature-controlled environments. Scrubbing with potassium permanganate is taking up space and needs service and renewal of the used potassium permanganate. Chemical reactions with ozone require ozone generators for supplying ozone and further require decomposition of unreacted ozone. The production and subsequent removal of excess ozone constitutes an undesirable waste of resources. Further, reefer containers and refrigerators have fixed dimensions. Accordingly, any space-requiring devices for cleaning the air and/or surfaces of the storing system will influence the storage capacity in a negative way. Thus, there is a need for a more space- and energy-efficient and cost-effective method for decomposition of organic compounds in cooled storing systems.
Photo catalytic activity is the ability of a material to create an electron hole pair as a result of exposure to ultraviolet radiation. The resulting free-radicals are very efficient oxidizers of organic matter.
The use of bandgab semiconductors such as TiO2, ZnO, ZrO2, CdS, etc. and their various modified forms as photo catalysts is well known in the prior art. Photo catalytic activity in TiO2 has been extensively studied because of its potential use in sterilization, sanitation, and remediation applications.
Two crystalline forms of TiO2 have photo catalytic activity, anatase and rutile. Anatase has a band gap of 3.2 eV and for rutile it is 3.0 eV. Anatase crystalline form has been found to be the most active and is readily excited upon exposure to near UV radiation. The action spectrum for anatase shows a sharp decrease in activity above about 385 nm with optimum wavelengths of approximately 254 nm producing electron/hole (e−/h+) pairs on the semiconductor surface.
The photo catalytic process includes chemical steps that produce reactive species. The steps include formation of i.a. the following species: hydroxyl radical, hydrogen peroxide, superoxide, conduction band electron, and valence band hole.
The recombination of e−/h+ pairs has the resulting effect of reducing the process quantum efficiency. The recombination can occur either between the energy bands or on the semiconductor surface.
It has long been recognized that certain materials such as noble metals (e.g. Pt, Pd, Au and Ag) and some metal oxides (e.g. RuO2, WO3, and SiO2) facilitate electron transfer and prolong the length of time electrons and holes remain segregated. The electrons and holes act as strong reducing and oxidizing agents that cause breakdown of the target compounds (ethylene, formaldehyde and ozone etc.) via formation of active radicals on the photo-catalyst surface. The photo-catalytic process is dependent on water i.e. from the humidity in the air.
Electron-Hole Pair Formation:TiO2+hν→TiO2−+OH.(or TiO2+)(conduction band electron and valence band hole)
Oxidation of Organic Compounds:OH.+O2+CnOmH(2n−2m+2)→→→nCO2+(n−m+1)H2O
Oxidation of Ethylene:3O2+C2H4→2CO2+2H2O
It has been described in CN201312536U to use photo catalysis for ethylene removal in refrigerated containers. However, this system is arranged in a dedicated and space demanding reaction tank inside the container, thus reducing the storage capacity and further requiring an increased amount of energy to circulate air into the reaction tank.
It is a first object of the present invention to provide a cooled storing system for photo catalytic decomposition of ethylene that is space- and energy-efficient and/or cost-effective.
It is another object of the present invention to provide a cooled storing system for photo catalytic decomposition of ethylene that is simple and easily maintained.
It is a further object of the present invention to provide a cooled storing system for photo catalytic decomposition of ethylene, wherein the photo catalytic surfaces are substantially self-cleaning.
It is a further object of the present invention to provide a method for photo-catalytic decomposition of organic compounds contained in a cooled storing system, which method is space- and energy-efficient and/or cost-effective.